Population sampling for research in psychology

Target population
When psychologists carry out research they do it with the intention of making the results apply to as many people as possible. The problem is that there are so many ways in which people differ from one another (individual differences such as age, culture, gender, intelligence level, etc) that it is impossible to have enough representatives of every possible type of person. To get round this problem, psychologists instead choose a certain population that their research will be applicable to (e.g. British sixth form students, single mothers aged 20-30, or middle aged men employed in manual jobs) - this population is called the target population.

It would be impractical to ask every member of a target population to take part in the research project as the target population could number many thousands of people. Instead, psychology researchers select a small number of members of the target population to participate, and they assume the results from the small selection (called a sample) will apply (generalise) to the rest of the target population.

Sampling methods
Random sampling
Random sampling involves selecting members of the target population in a random way. This can be done using a computerised random number generator, or even by simply drawing names out of a hat.
  • Advantages of random sampling are that every member of the target population has an equal chance of selection, and that there is no experimenter bias in who takes part.
  • The main disadvantage of random sampling are that even a random sample may not truly represent the target population (e.g. a random sample of 12 people may by chance all be the same gender).

Opportunity sampling
An opportunity sample is one that happens to already exist somewhere. An example could be year 12 students studying psychology as a ready-made group of these will exist in most sixth forms. Another example might be children already at a day care centre.
  • The main advantage of opportunity sampling are that it is quick and easy as the sample already exists.
  • The main disadvantage is that the opportunity sample is biased because the members of it have self selected and are all similar in at least one way, therefore any results will only be truly generalisable to that specific group of people.

Volunteer sampling
Volunteer samples are ones in which the participants have put themselves forward as research candidates. Researchers obtain volunteer samples by advertising on posters or in newspapers.
  • The main advantage of a volunteer sample is that participants will all be happy and willing to participate, and they will not feel coerced in any way.
  • The main disadvantage of an opportunity sample is that it will be biased towards a certain type of person as only people with a personal interest in the research topic will volunteer. The sample will not therefore be truly representative of the target population.

A Level exam tips
Answering exam questions (PSYA1 AQA A specification)
These tend to be 1 or 2 mark questions focused on showing you understand how to select participants using a sampling method, and that you know strengths and weaknesses of that sampling method. You need to be able to identify an appropriate sampling method for an example research study, and you need to state and elaborate on 1 strength and 1 weakness.